Ion source with reduced emittance

ABSTRACT

An ion source modified by an additional electrode, designated as a scraper electrode, positioned between the anode and extractor electrode, having an aperture slightly less than the source plasma aperture, and adapted to carry a pre-determined voltage in reference to the anode. It has been found that the scraper electrode intercepts and removes ions at the periphery of the beam cross-sectional area and this is the region that contains ions that contribute most greatly to the increased emittance. The central, non-intercepted cross-sectional area therefore has a much reduced emittance. The voltage on the scraper electrode should be such as to not disturb or distract the electric field between anode and extractor electrode.

United States Patent [1 1 Ormrod 1 Oct. 23, 1973 4] oN SOURCE WITH REDUCED EMITTANCE [75] Inventor: John H. Ormrod, Deep River,

Ontario, Canada- [73] Assignee: Atomic Energy of Canada Limited,

' Ottawa, Ontario, Canada [22] Filed: Oct. 24, 1972 [2]] Appl. No.: 299,763

[52] [1.8. Ci. 313/63, 313/230 [51'] int. Cl. H05h 7/00 [58] Field of Search 313/63, 230

Primary Examiner-Roy Lake Assistant ExaminerDarwin R. Hostetter Attorney-James R. Hughes [57] ABSTRACT An ion source modified by an additional electrode, designated as a scraper electrode, positioned between the anode and extractor electrode, having an aperture slightly less than the source plasma aperture, and adapted to carry a pre-determined voltage in reference to the anode. It has been found that the scraper electrode intercepts and removes ions at the periphery of the beam cross-sectional area and this is the region that contains ions that contribute most greatly to the increased emittance. The central, non-intercepted cross-sectional area therefore has a much reduced emittance. The voltage on the scraper electrode should be such as to not disturb or distract the electric field between anode and extractor electrode.

4 Claims, 7 Drawing Figures TO POWER SUPPLY PATENTEUOU23 i975 3767 95 sum 10F 5 TO POWER SUPPLY FIG.|

PATENTEI] URI 23 I975 SHEET S UF 5 QQQE l lll'llll l llllllll Illlll I 'Illllll 'Illllll 1 llllllll l [IIIIIII Illlll I l llllllll l Illlllll ION SOURCE WITH REDUCED EMITTANCE This invention relates to an ion source and more particularly to an ion source that has reduced source emittance.

The quality of an ion beam is described by its emittance which is a measure of the ability of the beam to traverse a given aperture. This parameter of the ion beam is defined as the populated area in a two dimensional transverse phase-space. It is a measure of the randomness or confusion in the transverse motion of the individual ions that make up the beam; the less confusion, the smaller the emittance, the better the beam. The emittance is an invariant of the motion and is an important parameter in ion optics calculations of beam transport. It should be stressed that emittance of a beam is not directly related to consideration of divergence or collimation and cannot normally be corrected or improved by focussing or collimating devices.

It is an object of the present invention to provide an ion source in which the source emittance has been reduced.

This and other objects of the invention are achieved by an ion source modified by an additional electrode, designated as a scraper electrode, positioned between the anode and extractor electrode, having an aperture slightly less than the source plasma aperture, and adapted to carry a pre-determined voltage in reference to the anode. It has been found that the scraper electrode intercepts and removes ions at the periphery of the beam cross-sectional area and this is the region that contains ions that contribute most greatly to the increased emittance. The central, non-intercepted crosssectional area therefore has a much reduced emittance. The voltage on the scraper electrode should be such as to not disturb or distract the electric field between anode and extractor electrode.

In drawings which illustrate an embodiment of the invention, g I

FIG. 1 is a cross-sectional view of a typical ion source of the duoplasmatron type to which the invention may be applied,

FIG. 2 is a cross-section view measuring unit,

FIG. 3 is a typical result obtained from the emittance measuring unit, I

FIG. 4 is a cross-section of the beam forming portion of the ion source with a scraper electrode, and

FIGS. 5A, 5B and 5C are experimental results obtained from a source incorporating a scraper electrode and for various potentials applied to the scraper.

Referring to FIG. 1, a typical duoplasmatron ion source for use in an accelerator producing a 100 mA dc proton beam is shown. This source is typical of ion sources which are devices for producing suitable plasmas with an electric field applied to extract the ions. A glass vacuum envelope contains the ion source made up of cathode 11, an intermediate electrode 12 (iron), a surrounding coil 13 (e.g. 3000 turns) for producing a magnetic field tocontain the plasma, anode 14 (copper), ceramic liner l5, and plasma aperture plate 16 all contained in a source containing can 17. The ceramic liner defines a funnel-shaped'region usually designated as the expansion cup 18. An extractor of a two-slit emittance electrode 19 carrying a high voltage in relation to the Good emittance is obtained from a plane interface between the plasma in the source and the extracted beam. The plasma-beam interface is usually not plane because of plasma density fluctuations, expecially the ratification near the boundary of the source aperture. For a source of the type shown in figure 1 the emittance may be readily measured by means of an apparatus such as shown in FIG. 2 involving two slits, a principal slit 20 and an analyzing slit 21. A beam stop 22 intercepts all the beam 23 except the thin ribbon 24 transmitted by the principal slit located at position x. Downstream the analyzing slit scans the ribbon which has spread in the x-direction due to a component (P of momentum. A complete emittance pattern may be obtained by P -scans (moving the analyzing slit) for about a dozen x-positions across the beam. The beam current to analyze the scan is measured by means of a Faraday cup 25 and an output current measuring device (not shown). The emittance has a lower limit determined by the diameter of the source and the transverse'momentum distribution of the ions in the plasma, however, the shape of the plasma-beam interface is quite often the dominating factor. A plane plasmabeam interface is ideal but this condition can only be realized if the plasma density near the interface is uniform. When a plasma comes in contact with a wall, a sheath is formed between the two with the plasma density decreasing across the sheath. Such a sheath is formed at the edge of the defining aperture 16 of a source resulting in an annulus of lower density plasma. The plasma-beam interface recedes into the source plasma expansion cup 18 at this annulus and the adjacent equipotential surfaces of the extracting field ,assume a curvature of the plasma boundary which is shown in FIG. 3 as obtained by the apparatus of FIG. 2. The initial ion acceleration is normal to these equipotentials and the resulting emittance is characterized by the triple peaked (P -scans) shown as A, B and C. The widths of the individual peaks is ameasure of "the inherent transverse momentum distribution of the ions in the plasma; the satellite peaks (B and C) arise from the shape of the plasma beam interface 26 andincrease the effective emittance considerably.-

FIG. 4 illustrates the means according to the invention in which the emittance of a source may be lowered. The electrode arrangement of FIG. 1 is shown (with the same elements similarly identified) but in this case an additional electrode called a scraper electrode is positioned immediately downstream from the source plasma aperture. The scraper electrode has an aperture diameter (11),) slightly less than the plasma aperture diameter and must be near the plasma surface to intercept the inward directed component from the annulus before it mixes with the rest of the beam. The scraper electrode should be at a potential to least disturb the plasma surface and as the required voltage increases as the four-thirds power of the distance from the plasma surface, it is preferable to keep the spacing as small as practicable so that the voltage level required will be small and thus the power dissipation by the electrode.

FIGS. 5A, 5B and 5C show examples in graphical form of a transverse momentum scan log current vs transverse momentum, for three values of scraper electrode potential i.e., the scraper at 0 volts (equivalent to no scraper), at volts, and at 300 volts. It will be seen that as the potential is increased (to 300 volts),

the satellite peaks, in these cases, shoulders decrease over thirty-fold to approximately 1 percent of the main peak, and the width of the distribution at percent of peak weight decreases approxiamtely 3-fold which is also the decrease in the emittance. These results were obtained for a 6mm diameter source.

What is claimed is:

1. An ion source of the type having a chamber for containing an ionized gas plasma, an aperture in a wall of said chamber, and extractor electrode positioned external to said aperture and adapted to carry an electrical potential such that ions are extracted from the plasma and formed into an ion beam, the improvement comprising a scraper electrode positioned adjacent the said chamber aperture and in spaced relationship towards the extractor electrode, said scraper electrode containing an aperture whose diameter is slightly less than the aperture in the chamber wall and effective to intercept and remove the outer portion of the ions in the said beam and thus reduce the emittance of the beam.

2. An ion source as in claim 1 wherein the scraper electrode is adapted to carry a potential in relation to that of the chamber and the extractor electrode such that the electric field distribution in the region between extractor and chamber is not greatly difi'erent to that which would occur if the scraper electrode were not present.

3. An ion source having reduced emittance comprising:

a. an ionization chamber,

b. means in said chamber for ionizing gas therein,

0. a plasma forming chamber connected to the ionization chamber,

d. an anode,

e. an extractor electrode for extracting ions from the plasma via an aperture in a plasma aperture plate associated with said anode and forming an ion beam therewith, and

f. a scraper electrode positioned adjacent the plasma aperture plate towards the extractor electrode, said scraper electrode containing an aperture whose diameter is slightly less than that of the plasma aperture plate and effective to intercept and remove the outer portion of the ions in the said beam, said ions being those that have transverse momentum components that give rise to the satellite peaks in the emittance.

4. An ion source as in claim 1 wherein the scraper electrode is adapted to carry a potential in relation to that of the anode and extractor electrode such that the electric field distribution in the region between anode and extractor is not greatly different to that which would occur if the scraper electrode were not present. 

1. An ion source of the type having a chamber for containing an ionized gas plasma, an aperture in a wall of said chamber, and extractor electrode positioned external to said aperture and adapted to carry an electrical potential such that ions are extracted from the plasma and formed into an ion beam, the improvement comprising a scraper electrode positioned adjacent the said chamber aperture and in spaced relationship towards the extractor electrode, said scraper electrode containing an aperture whose diameter is slightly less than the aperture in the chamber wall and effective to intercept and remove the outer portion of the ions in the said beam and thus reduce the emittance of the beam.
 2. An ion source as in claim 1 wherein the scraper electrode is adapted to carry a potential in relation to that of the chamber and the extractor electrode such that the electric field distribution in the region between extractor and chamber is not greatly different to that which would occur if the scraper electrode were not present.
 3. An ion source having reduced emittance comprising: a. an ionization chamber, b. means in said chamber for ionizing gas therein, c. a plasma forming chamber connected to the ionization chamber, d. an anode, e. an extractor electrode for extracting ions from the plasma via an aperture in a plasma aperture plate associated with said anode and forming an ion beam therewith, and f. a scraper electrode positioned adjacent the plasma aperture plate towards the extractor electrode, said scraper electrode containing an aperture whose diameter is slightly less than that of the plasma aperture plate and effective to intercept and remove the outer portion of the ions in the said beam, said ions being those that have transverse momentum components that give rise to the satellite peaks in the emittance.
 4. An ion source as in claim 1 wherein the scraper electrode is adapted to carry a potential in relation to that of the anode and extractor electrode such that the electric field distribution in the region between anode and extractor is not greatly different to that which would occur if the scraper electrode were not present. 